“Athletes don’t get coaching on this,” he says. “They get fitness coaching. They get nutritional coaching. They don’t get sleep coaching.” Among other teams, he has consulted with the NBA’s Minnesota Timberwolves and Portland Trail Blazers, Major League Baseball’s Milwaukee Brewers, and nearly every pro sports team in the greater Boston area. When the New England Patriots were preparing to fly to London in 2012 for the NFL’s yearly overseas regular-season game, the team physicians sought Czeisler’s expertise before they left. And 15 minutes into our conversation at Harvard, he left the room to take a call from a Major League Baseball team he preferred not to name, which was working on schedule preparations for the 2014 season.

For Czeisler, however, the importance of optimizing sleep extends beyond sports. It’s a public health issue, he says, calling sleep the “third pillar” of good health, alongside diet and exercise. If you don’t get enough over time, he says, you run the risk of ailments like heart disease, diabetes, even cancer. More is always better, he says, because research has proven that it is impossible to get too much sleep. Czeisler recommends seven or eight hours a night for regular people, and closer to nine for high-performance athletes.

If you think you can power through on anything less—as a lot of people do—he points to a series of breakthrough studies led by a Danish biologist whose findings were published in 2013 in the journal Science. The researchers zeroed in on one of the fundamental purposes of sleep: to cleanse your brain.

“We replace cells everywhere all the time, but we keep our brain cells for a lifetime because the connections are so complicated, and to retain our memories,” says Czeisler. “So if you’re going to repair the brain, you’ve got to bring the system offline.” The Danish team offered evidence that your spinal fluid flushes out the toxic buildups on your cells during sleep. “During the process, the space between the [brain] cells becomes larger,” says Czeisler. “There is a structural change, with things moving in relation to one another. It would be as if all the buildings in Manhattan shrank, and the alleys and streets got bigger for the garbage trucks. It was remarkable.”

The message is obvious—it’s important to get enough sleep; but for him, it has become a mission. “For someone like Czeisler, it’s not necessarily about individual patients, it’s about everybody,” says Matthew Wolf-Meyer, Ph.D., associate professor of anthropology at the University of California, Santa Cruz, and author of The Slumbering Masses: Sleep, Medicine, and Modern American Life. “He’s using athletes, astronauts—exceptional cases—in order to get people to think about sleep, and to maximize sleep for everybody; he’s working to popularize the science of sleep in ways that clinicians and scientists don’t really do.”

In January, Czeisler traveled to Davos, Switzerland, with Huffington Post founder Arianna Huffington, to attend the World Economic Forum, the annual convention of the world’s top political and business leaders. In 2013, he testified in the wrongful death trial of the late Michael Jackson, arguing on behalf of the pop star’s mother and children that the troubled musician had suffered from “total sleep deprivation over a chronic period” by the time he died.

“Chuck has the most effective communication and lecture skills,” says David Dinges, Ph.D., chief of the division of sleep and chronobiology at the University of Pennsylvania’s psychiatry department. “He is a giant. He’s been an extraordinary influence in his ability not only to make these big discoveries in the field of sleep, but to translate them to the public in a way that people actually understand.”

Czeisler’s pupils now include several multimillionaire professional athletes—guys not always up for sitting through classroom-style lectures about the nature of REM sleep or instructions on when to go to bed. “There’s obviously a learning curve involved, when Chuck is coming from the academic world to the sports world and trying to educate players,” says Ed Lacerte, head trainer for the Boston Celtics. “But it’s easy to work with Chuck—or ‘Doctor Z,’ as I like to call him. He’s been able to sit down with our players one-on-one. The athletes listen to him. We’ve had players go to his lab for sleep studies, and he’s actually gone into their homes to work with them. He’s been accepted fine. His height has certainly helped—I can tell you that.”

Czeisler is 6’4”, with wispy gray hair and an old-fashioned mustache. He is a polite and enthusiastic talker, and a prolific doodler. When you meet him, you discover that he seldom struggles to find the right words to say—but when he does, he squints into the distance as if the rest of his thought lies waiting to be plucked from across the room. As we chat, his smartphone lights up with e-mails and texts. On one occasion he checks his screen. “I guess I did make some friends along the way,” he says, with a squint and a smile. “That was Shaq [former NBA star Shaquille O’Neal] . He’s a really nice guy.”